The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present invention is in the technical field of optical imaging. More particularly, the present invention is in the technical field of Optical Coherence Tomography. More particularly, the present invention is in the technical field of Full-Field Optical Coherence Tomography with a probe.
Optical Coherence Tomography (OCT) is an imaging technique based on low coherence interferometry (see for example J. G. Fujimoto et al., Optical biopsy and imaging using optical coherence tomography, Nature Medicine 1, 970-972 (1995)). Because of its good sectioning ability and a typical 10 micrometer-scale resolution it was found to be an efficient tool for in-depth imaging of biological tissues. In contrast with most of the available OCT approaches, e.g. time domain OCT or Fourier domain OCT, Full-Field OCT (FF-OCT) directly takes “en face” high resolution images (typically 1 μm, isotropic) using two-dimensional (2D) detectors, thus eliminating the need for lateral x y scanning (see for example A. Dubois et al., High-resolution full-field optical coherence tomography with a Linnik microscope, Applied Optics 41, 805-812 (2002)).
A typical FF-OCT setup with a Linnik interferometer and a modulation of the reference path using a piezoelectric (PZT) oscillator is represented on FIG. 1. It relies on the use of thermal sources or arcs or LEDs that are spatially incoherent, coupled to an interference microscope, for example a Michelson in the Linnik configuration as shown on FIG. 1. The FF-OCT system 100 comprises a source of partially coherent light 101, e.g. an halogen light source, a beam splitter 102, e.g. a non-polarizing beam splitter cube, defining two interferometric arms. For a Linnik configuration, both arms include a microscope objective of the same characteristics 103 and 104. In one arm a uniform reflective surface 105 is positioned at the focal plane of the objective and linked to an oscillator 111, allowing modulation of the optical path length of the reference arm, e.g. a piezo electric transducer. In the other arm the volume and scattering sample 106 is positioned at the focal plane of the objective 103. An adjustable dispersion balance system is included in both arms, e.g. rotating glass plates 109 and 110. A tube lens 107 is placed at the output of the interferometer in order to conjugate the focal planes of both objectives 103 and 104 with a multichannel detector 108.
All OCT systems have a limited maximum imaging depth in tissues of typically one to two millimeters due to absorption and scattering of light by the biological structures. For imaging of internal organs for example a probe is thus required.
Classical OCT, i.e. non Full-Field OCT, systems with probe are optical fibre versions of the Michelson interferometer, where 2D images are acquired with point-to-point scanning at the tip of the probe (see for example P. H. Tran et al., In vivo endoscopic optical coherence tomography by use of a rotational microelectromechanical system probe, Optics Letters 29, 1236-1238 (2004)). Such scanning systems require advanced miniaturized mechanical systems at the tip of the probe, as well as electric supply. The advantage of a FF-OCT setup is that it does not require any scanning since all images are taken “en face”.
A FF-OCT system with probe has to address the problem of keeping the performances of FF-OCT in a set-up using a miniaturized, medically safe probe. The probe part cannot be directly integrated as an arm of the Linnik interferometer because its use would cause perturbations and damage the interference signal. Indeed if the probe comprises an optical fiber or fiber bundle, bends and twists in the fibers during in vivo measurements will create differences between the states of polarization of light in the reference and object arms, thus distorting the signal. Moreover it would also require to set identical probes in both arms of the Linnik interferometer, which would induce very large optical path lengths difficult to balance. On the contrary, in a system with two interferometers the probe is not part of an interferometer arm and is only used to transport an image. It is thus entirely passive and insensitive to its environment. Such a system is to privilege for in situ imaging, where one needs a system which is able to image outer or inner parts of the body that are difficult to reach. Thus FF-OCT systems with a probe use two coupled interferometers.